It is important to measure a moisture content in the living body of a subject. Dehydration in the living body is a pathological condition in which the moisture content in the living body decreases, and is a symptom that develops frequently in daily life, and in particular, develops more often when a subject is doing an exercise or the atmospheric temperature is high because a large amount of water is excreted from the body as a result of perspiration or a temperature rise. In particular, since in many case, older persons have a decreased ability to retain water in the living body, it is said that older persons are more likely to have dehydration than ordinary healthy persons.
In general, when a person gets older, the volume of muscles that store water decreases, the urinary volume increases due to a decrease in the kidney function, the ability to sense thirst in the mouth decreases due to dulled sensitivity, and the moisture content required for the cells decreases. When the dehydration is left without any treatment, the dehydration may cause and even develop into severe symptoms. Infants may also suffer from the same dehydration. Although the moisture content of infants generally large, the infants cannot appropriately appeal for the supply of water by themselves and may have dehydration since the persons who care the infants recognize it too late.
In general, it is said that a disorder in temperature regulation occurs when more than 2% of weight of moisture in the living body is lost. The disorder in temperature regulation causes such a vicious circle that it causes an increase in temperature, which in turn causes a reduction in the moisture content in the living body, and finally results in a pathological condition called a heat illness. Heat illnesses include heat cramps, heat exhaustion, heat strokes, which may sometimes cause organ disorder in the entire body. Thus, it is preferable to accurately detect dehydration to prevent such a danger that leads to heat illnesses.
As a device for detecting dehydration, an apparatus that measures a body impedance using such a device having handles to be held by both hands to calculate a moisture content from the measured impedance is known (see Patent Documents 1 to 3).
As another device for detecting dehydration, an oral moisture meter or the like that measures a moisture content in the mouth such as lingual mucosa, buccal mucosa or palate is known (see Patent Documents 4 to 6).
As a method for measuring moisture content in the skin, an in-vitro mass method, a Karl Fischer method, an in-vivo ATR spectroscopy, and a high-frequency impedance method and electrical conductivity method which are simpler in-vivo measurement methods are generally used.